Mnemonic for ‘Halt and Catch Fire’, any of several
undocumented and semi-mythical machine instructions with destructive
side-effects, supposedly included for test purposes on several well-known
architectures going as far back as the IBM 360. The MC6800 microprocessor
was the first for which an HCF opcode became widely known. This
instruction caused the processor to toggle a subset
of the bus lines as rapidly as it could; in some configurations this could
actually cause lines to burn up. Compare
killer poke.